CA1167905A - Grounded housing switching shock hazard reduction by surge impedance - Google Patents

Abstract

49,280 ABSTRACT OF THE DISCLOSURE
Gas insulated electrical apparatus includes a high voltage conductor disposed within a grounded metallic housing, with an insulating gas disposed therebetween. An earthing switch connects the high voltage conductor to ground, and an impedance element is connected in series with the earthing switch.

Description

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1 49,280 EARTHING DEVICE
BACKGROUND OF THE INVENTION
This invention relates to an earthing device for an enclosed, gas insulated electrical apparatus includiny as an insulating medium, a gas high in dielectric strength, for example, SF6 gas or the like.
Recently, metal-clad, gas insulated substations using compressed gases such as SF6 gas or the like high in dielectric strength as compared with air, have been put into practical use. Those gas insul-ated substations are composed generally of circuit interrupters, disconnecting switches, earthing switches, ducted buses, lightning arresters and the like. One of the more important pieces of e~uipment is the earthing switch, which provides a ready path to ground for residual voltage trapped on the conducting parts of the substation, thereby increasing safety for station personnel working in the vicinity.
SUMMARY OF THE INVENTION
In accordance with this invention, an earthing device is provided which includes a metallic housing containing an insulating gas, a high voltage electrode connected to ground through an earthing switch, and an impedance element connected in series with the earthing switch.
BRIEF DESCRIPTION OF THE DRAWINGS
Reference is now made to the Description of the Preferred Embodiment, illustrated in the accompanying drawings, in which:
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2 4g,2~0 Figure 1 is a sectional view illustrating a conventional prior art gas insulated substationi Fig. 2 is an equivalent circuit for steep wave surges in conventional prior art gas insulated substa tions;
Fig. 3 is a sectional view illustrating one embodiment of an earthing device according to the present invention;
Fig. 4 is an equivalent circuit for steep wave surges in a gas insulated substation including the earth-ing device o the presen-t invention; and Figs. 5 through 7 are sectional view-s illustrat-ing other embodiments according to the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMFNT
Referring now more particularly to Fig. 1, therein is illustrated a simplified prior art gas insu-lated substation 110. The substation 110 comprises an earthed tank 1 serving as a metallic housing, a circuit interrupter 2; a disconnecting switch 3; and an earthing switch 4. Central conductors 51' 52' and 53 (in collect-ive reference to which the numeral 5 is used) are on the high voltage side, and leads~ ,' ~ (referred to collect-ively by the reference numeral (6)) are utilized for connecting the earthing switch 4 between the central conductors 53 and the earthing tank 1. The space 7 within the earthed tank 1 is charged with an electrically nega-tive gas such as SF6 gas or the like. This gas insulated substation 110 is connected to a cable 112 or the like through an insulating fitting 8, the cable 112 consisting of a central conductor 9 and an earthed sheath 10. Leads 11, 12 are used for earthing the metallic housing 1 and the earthed sheath (10) of the cable 112, and are earthed at a position deep below the ground 13.
Next the operation is described. In the normal power transmission state of the gas insulated substation 110 the circuit interrupter 2 and the disconnec~ing switch ' `; .

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3 are in their closed positions, the earthing s~litch 4 is in its open position and a load current flows in each of the central conductors 51' 52' 53 and 9. Since the earth-ed tank 1 and earthed sheath 10 at that time are earthed5 through the earthing leads 11 and 12, respectively, they are held at earth or ground potential and free from danger because even if a human being 14 were to touch them, he or she would receive no electric shock.
When it is necessary to stop the power trans-mission of this gas insulated substation 110 for some reason, such as insp~ction or the switching of a trans-mission circuit, the central conductors 53 and 9 have been arranged to be earthed by interrupting the circuit current with the circuit interrupter 2, opening the disconnecting switch 3, and c]osing the earthing switch 4. If the cable side, that is, the load side 114 has a null or a low load upon this interruption of the circuit current, the inter-ruption effected by the circuit interrupter 2 results in the interruption of a charging current for the cable 112.
After the interruption, the central conductor 53 has remaining thereon a high DC voltage corresponding to the peak of the circuit voltage. This remaining voltage has a discharge time constant which is normally very long, for example, several tens of hours. Accordingly, in the above-mentioned earthing operation, the earthing switch 4 earths the conductor 53 having a voltage in the order of the substantial peak value of the circuit voltage. Under these circumstances, the closure of the earthing switch 4 puts the earthed tank 1 in a floating potential condition, although the duration thereof is very short, being in the order of microseconds. When a human being 14 touches the earthed tank 1 during switching, he or she may receive an electric shock, which is very dangerous. A cause for the development of such an abnormal voltage is that when the earthing switch 4 is closed with residual voltage in the cable 112, the electric charge accumulated in the cables :,-~ .

4 49,2~0 is propagated as a travelliny wave, but the insulatingfitting 8 acts an an impedance non-matching portion, or the earthing leads 11 and 12 may act as impedances or surge impedances. Figure 2 shows in a very simplified manner an equivalent circuit for this steep surge wave with a very short duration. Zl is a surge impedance possessed by the central conductor 53 in the gas insulated substation 110 with respect to the earthed tank 1, Z2 is a surge impedance possessed by the earthed tank 1 with respect to ground 13; Z3 is a surge impedance within the cable 112, and Z4 is a surge impedance of the earthed cable sheath 10 relative to ground 13. Z5 and Z6 are impedances possessed by the earthing leads 11 and 12, respectively. As will be seen from Fig. 2, after a point A corresponding to the central conductor 9 of the cable 112 has been charged, the closure of the earthing switch 4 causes a potential at a point B corresponding to the earthed tank 1 adjacent to the earthing switch 4 to rise from ground by a voltage drop across the impedances Z2 and Z5. However,~ a time interval of propagation of a travel-ling wave through the Z2 and Z5 portions is in the order of microseconds and very short so that the high voltage is not sustained for a long time.
However, the operator might encounter danger, even though the duration is short, because such abnormal voltage is developed during switching.
In order to eliminate such disadvantages as described above, the present invention has been made, and it aims at the construction of an earthing device having an impedance such as a resistance or an inductance insert-ed serially therein and providing a device capable of preventing the rise in the potential of the earthed tank which occurs upon operating the earthing switch.

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., 4g,2~0 One embodiment of the present invention is described with re~erence to Fig. 3.
~ ig. 3 is a sectional view illustrating one embodiment of an earthing device accordiny to the present invention. In Fig. 3, the components corresponding to Fig. 1 are designated by corresponding reference numerals.
This is also true in the case of the succeeding Figures 4-7.
Fig. 3 is a device 120 having a surge suppress-ing impedance Z7 such as a resistance or an inductance inserted into the gas insulated substation 110 by connect-ing it between the earthing switch 4 and the earthed tank 1 by means of leads 63 and 64.
Next the operation of this embodiment is des-cribed. In the normal power transmission state the earth-ing switch 4 is in the open position so that this embodi-ment is exactly the same as the prior art practice shown in Fig. 1.
Next, the circumstances when the circuit has no load, the circuit interrupter 2 interrupts only the charg-ing current of the cable 112 and, after the interruption, a residual voltage of the order of the peak value of the circuit voltage remains on the cable 112 and this is discharged through the earthing switch 4 are considered.
As shown in Fig. 3, a surge suppressing imped-ance Z7 such as a resistance or an inductance is connected between the earthing switch 4 and the earthed tank 1 so that the potential rise at the earthed tank 1 portion can be suppressed.
Fig. 4 shows a comparison of the embodiment shown in Fig. 3 with the prior art example of Fig. 2 with respect to an e~uivalent circuit for a steep surge wave.
Z7 is an impedance for suppressing surges. From Fig. 4, it is seen that, if the earthing switch 4 is closed with a point A corresponding to the cable conductor 9 left charged, the potential at point B corresponding to the ' ,~

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6 4g,2~0 earthed tank 1 adjacent ~o the earthing sWitch 4 rises from ground by a voltage drop across impedances Z2 and Z5.
In these circumstances, as serially connected surge sup-pressing impedance Z7 limits the current flowing, the potential rise at the point B is reduced by a correspond-ing amount. In this case, when the impedance Z7 is high-er, the flow of current is made smaller, and therefore thepotential rise at the earthed tank 1 can be made small.
However, no matter how large, the residual voltage dis-charge time constant should be a value not exceeding about1 second.
While this embodiment illustrates an example of the insertion of an impedance Z7 in series with the earth-ing switch ~, leads 65 and 66 may be used to dispose a second switch 15 in parallel with impedance Z7, as illu-strated in Fig. 5, with the second switch 15 arranged tobe closed at a specified time after the closure of the earthing switch 4, so that abnormal voltages are suppress-ed and more reliable earthing can be achieved, and the generation of abnormal voltages can be similarly suppress-ed.
Also as another embodiment, as shown in Fig. 6, a ferromagnetic member 16 such as a hollow cylindrical iron core or the like is disposed around the lead 62 thereby increasing the inductance of this lead 62 and enabling the suppression of surges. This method is high in reliability because the connection is effected by only the lead 62 and when the steady state is reached in which the influence of the inductance is negligible. In this case if an iron cora of a material such as a ferrite core of good fre~uency characteristics and high relative mag-netic permeability is used, the effect achieved will be great.

.~-7 ~9,280 Also, as yet another embodiment, the earthing switch 4 may be constructed as shown, for example, in Fig.
7, so as to complete the two-stage closing operation of Fig. 4 in one operation. In the exarnple shown in Fig. 7, the surge suppressing impedance Z7 is provided at both ends with contactors 17 and 18 connected thereto through leads 67 and 68, respectively. At the time of the earth-ing operation, an electrically conducting earthing rod 19connected to the earthed tank 1 is moved. The earthing rod 19 first contacts the contactor 17 to earth the same and suppress surge, after which it contacts the contactor 18 to effect full earthing.
As described above, according to the present invention, an earthing device for a gas insulated elec-trical apparatus is constructed by inserting serially animpedance into an earthing switch. Therefore, an abnormal voltage occurring on earthed tank upon operating the earthing device can be suppressed so that the effect of increasing safety during operation, is provided.

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Claims (4)

8 49,280 CLAIMS:

1. Gas insulated electric apparatus comprising:
a grounded metallic housing having an insulating gas disposed therein;
a high voltage conductor disposed within said housing;
an earthing switch connecting said high voltage conductor to said metallic housing; and an impedance element connected in series with said earthing switch.

2. The apparatus according to claim 1 wherein said earthing switch is disposed between said high voltage conductor and said metallic housing through connecting leads and the closing operation thereof is attended with the electrical connection of said high voltage conductor to the metallic housing.

3. The apparatus according to claim 2 wherein the impedance element consists of a resistor or an in-ductor.

4. The apparatus according to claim 3 wherein the impedance element consists of a ferromagnetic member in the form of a hollow circular cylinder surrounding the connection lead connecting the earthing switch to the metallic housing.